能计算加减乘除表达式，求添加计算平方、三角函数的功能，在线等，急

// EX6_08.CPP

// A program to implement a calculator



#include <stdio.h>  // For input/output

#include <stdlib.h>	// For the exit() function

#include <ctype.h>  // For the isdigit() function

#include <string.h> // For the strcpy() function



void eatspaces(char * str);					// Function to eliminate blanks

double expr(char * str);					// Function evaluating an expression

double term(char * str, int * pindex);	    // Function analyzing a term

double number(char * str, int * pindex);    // Function to recognize a number

char * extract(char * str, int * index);    // Function to extract a substring



const int MAX = 80;			// Maximum expression length including '\0'



int main(void)

{

	char buffer[MAX];		// Input area for expression to be evaluated

	char c;

	int i;



	printf("Welcome to your friendly calculator.\n");

	printf("Enter an expression, or an empty line to quit.\n");



	for(;;)

	{

		i=0;

		scanf("%c",&c); // Read an input line

		while(c!='\n')

        {

            buffer[i++]=c;

            scanf("%c",&c);





		}

		buffer[i]='\0';



		eatspaces(buffer);						// Remove blanks from input



		if(!buffer[0])							// Empty line ends calculator

			 return 0;



		printf( "\t= %f\n\n",expr(buffer));		// Output value of expression

	}



}





// Function to eliminate blanks from a string

void eatspaces(char * str)

{

	int i=0;				 // 'Copy to' index to string

	int j=0;				 // 'Copy from' index to string



	while((*(str+i) = *(str+j++)) != '\0')	 // Loop while character copied is not \0

		if(*(str+i) != ' ')					 // Increment i as long as

			 i++;							 //	character is not a blank

	return;

}





// Function to evaluate an arithmetic expression

double expr(char * str)

{

	double value = 0;					// Store result here

	int index = 0;						// Keeps track of current character position



	value = term(str, &index);	        // Get first term



	for(;;)								// Infinite loop, all exits inside

	{

		switch(*(str+index++))	        // Choose action based on current character

		{

			 case '\0':					// We're at the end of the string

				return value;			//	so return what we have got



			 case '+':							// + found so add in the

				value += term(str, &index);		//	next term

				break;



			 case '-':							// - found so subtract

				value -= term(str, &index);		//	the next term

				break;



			 default:											 // If we reach here the string

				printf("Arrrgh!*#!! There's an error.\n");

				exit(1);

			}

	 }

}





// Function to get the value of a term

double term(char * str, int * pindex)

{

	double value = 0;					// Somewhere to accumulate the result



	value = number(str, pindex);		// Get the first number in the term



	// Loop as long as we have a good operator

	while((*(str+(*pindex))=='*')||(*(str+(*pindex))=='/'))

	{



		if(*(str+(*pindex))=='*')					// If it's multiply,

        {

            ++(*pindex);

			 value *= number(str, pindex);			//	multiply by next number

        }



		if(*(str+(*pindex))=='/')					// If it's divide,

        {

            ++(*pindex);

			 value /= number(str, pindex);			//	divide by next number

        }

	 }

	 return value;						 // We've finished, so return what we've got

}





// Function to recognize a number in a string

double number(char * str, int * pindex)

{

	double value = 0.0;						// Store the resulting value



	char * psubstr;                         // Pointer for substring

	if(*(str + (*pindex)) == '(')           // Start of parentheses

    {

        ++(*pindex);

        psubstr = extract(str, pindex);     // Extract substring in brackets

        value = expr(psubstr);              // Get the value of the substring

        return value;                       // Return substring value

    }



	while(isdigit(*(str+(*pindex))))		// Loop accumulating leading digits

		value=10*value + (*(str+(*pindex)++) - 48);



											// Not a digit when we get to here



	if(*(str+(*pindex))!='.')				// so check for decimal point

		return value;						// and if not, return value



	double factor = 1.0;					// Factor for decimal places

 	while(isdigit(*(str+(++(*pindex)))))	// Loop as long as we have digits

	{

		factor *= 0.1;								 // Decrease factor by factor of 10

		value=value + (*(str+(*pindex))-48)*factor;	 // Add decimal place

	}



	return value;										 // On loop exit we are done

}



// Function to extract a substring between parentheses

// (requires string.h)

char * extract(char * str, int * pindex)

{

    char buffer[MAX];           // Temporary space for substring

    char * pstr = NULL;         // Pointer to new string for return

    int numL = 0;               // Count of left parentheses found

    int bufindex = *pindex;     // Save starting value for index

    do

    {

        buffer[(*pindex) - bufindex] = *(str + (*pindex));

        switch(buffer[(*pindex) - bufindex])

        {

            case ')':

                if(numL == 0)

                {

                    buffer[(*pindex) - bufindex] = '\0';    // Replace ')' with '\0'

                    ++(*pindex);

                    pstr = (char *) malloc((*pindex) - bufindex + 1);

                    if (!pstr)

                    {

                        printf("Memory allocation failed, program terminated.") ;

                        exit(1);

                    }

                    strcpy(pstr, buffer);   // Copy substring to new memory

                    return pstr;            // Return substring in new memory

                }

                else

                    numL--;     // Reduce count of '(' to be matched

                break;

            case '(':

                    numL++;     // Increase count of '(' to be // matched

                    break;

        }

    } while(*(str + (*pindex)++) != '\0');      // Loop - don't overrun end of string

    printf("Ran off the end of the expression, must be bad input.\n");

    exit(1);

}